At this time, brief application of an electric shock to the heart muscle is the only viable method that has been shown to be an effective treatment for termination of the most lethal ventricular tachyarrhythmia, ventricular fibrillation. Recently, implantable rhythm management systems have been developed to automatically monitor the heart rhythm and deliver appropriate electrical therapy when the heart beat is too fast or too slow. When a bradyarrhythmia is detected, the device issues low voltage (.about.5 V) electrical stimuli to drive the heart at a slightly faster rate. When a tachyarrhythmia is detected, the device charges one or more capacitors to .about.700 V and then discharges the capacitors when the tachyarrhythmia condition is reconfirmed by the device. Such rhythm management systems are called implantable cardioverter defibrillators (ICDs).
The lead systems commonly connected to ICDs have a sensing and pacing electrode located at the tip of the lead located in the right ventricular apex and in the right atrium. Some but not all lead systems have another electrode in the superior vena cava. Contemporary ICDs utilize the metallic shell (or case) of the ICD as an electrode that is active during defibrillation shocks. Such an arrangement permits defibrillation with lower strength shocks since the electrical resistance during the shock is lower with the shell electrode compared to electrode configurations without the shell electrode. Typically, the superior vena cava electrode and the shell electrode are made electrically common. In this case, the electrode configuration is said to be RV.fwdarw.SVC+Shell. Often the shell electrode is referred to as the "can" electrode, since the ICD case is typically referred to by those skilled in the art as the "can."
U.S. Pat. No. 5,282,837 to Adams et al. describes, in FIG. 1 and accompanying text, an atrial defibrillator and method in which a lead 36 is inserted into the coronary sinus so that a first tip electrode 42 is within the coronary sinus adjacent the left ventricle, a second ring electrode 44 is within the coronary sinus beneath the left atrium, and the third electrode 46 within the right atrium or superior vena cava. The first electrode serves as a sensing electrode, the second electrode (still in the coronary sinus) serves as both a sensing and defibrillating electrode, and the third electrode serves as a sensing and defibrillating electrode (see Col. 5 line 57 to Col. 6 line 12).
U.S. Pat. No. 5,433,729 to Adams et al. describes, in FIG. 9 and accompanying text, a lead system 250 configured in accordance with that described above. A first (right ventricle) lead 252 includes an elongate large surface area electrode 256, a distal or tip sense electrode 258, and a ring or proximal sense electrode 260. Sense electrodes 258, 260 are positioned in and in contact with the wall of the right ventricle, and elongate electrode 256 is in the right atrium. A second (coronary sinus) lead 254 includes a tip, or distal sense electrode 264, a ring or proximal sense electrode 266, and a second elongate, large surface area electrode 262. Distal and proximal sense electrodes 264, 266 are both adjacent the left ventricle within the great vein, and elongate electrode 262 is within the coronary sinus beneath the left atrium. The right ventricle sense electrodes 258, 260 are coupled to inputs 50a, 50b of first sense amplifier 50; the great vein sense electrodes 264, 266 are coupled to inputs 52a, 52b of second sense amplifer 52. This is to provide sensing of the right ventricle and the left ventricle, and the non-coincident sensing of the depolarization activation waves. for synchronizing delivery of energy to the atria (see column 15 line 34 to column 16 line 54; column 5 lines 62-64).
U.S. Pat. No. 5,014,696 to Mehra describes an endocardial defibrillation electrode system in which a coronary sinus electrode extending from an area adjacent the opening of the coronary sinus and terminating in the great vein is used in combination with subcutaneous plate electrodes and with right ventricular electrodes. The coronary sinus electrode 78 encircles the left ventricle cavity 86 (Col. 5 lines 50-51; FIG. 5B). It is stated "it is important not to extend the electrode 78 downward through the great vein 80 toward the apex 79 of the heart" (col. 5 lines 28-30). U.S. Pat. No. 5,165,403 to Mehra (Medtronic, Inc.) describes an atrial defibrillation electrode 112 that is located "within the coronary sinus and the great cardiac vein."
U.S. Pat. No. 5,099,838 to Bardy describes a defibrillation electrode in the great vein that is used in combination with subcutaneous plate electrodes and with right ventricular electrodes (col. 1 line 65 to col. 2 line 2). With respect to the great vein electrode, it is stated at column 5, lines 20-33 therein: "When so mounted, the elongate defibrillation electrode 78 extends from a point adjacent the opening of the coronary sinus 74 and into the great vein 80. This provides a large surface area defibrillation electrode which is generally well spaced from the ventricular defibrillation electrode 74 and provides good current distribution in the area of the left ventricle 77. It is desireable to extend the electrode 78 around the heart as far as possible. However, it is important not to extend the electrode 78 downward through the great vein 80 toward the apex 79 of the heart, as this will bring the coronary sinus and right ventricular electrodes into close proximity to one another, interfering with proper current distribution. U.S. Pat. No. 5,193,535 to Bardy (filed Aug. 27, 1991) also describes a great vein electrode. At column 7, lines 31-35, it is stated: "The coronary sinus lead is provided with an elongated electrode located in the coronary sinus and great vein region at 112, extending around the heart until approximately the point at which the great vein turns downward toward the apex of the heart."
U.S. Pat. No. 5,431,683 to Bowald et al. describes a ventricular defibrillation electrode system in which an electrode is placed through the coronary sinus into a peripheral vein of the heart. The term "peripheral vein" is defined therein as to encompass "the venous side of the coronary vessels running between the base and the apex of the heart. The veins include the middle and small cardiac veins, and the portion of the great cardiac vein which runs between the base and apex of the heart. The definition of peripheral veins' used herein therefore excludes that portion of the great cardiac vein which runs along the base plane of the heart, which has been used [as] a site for electrode placement in prior art electrode systems." The electrodes are in the shape of a helix to apply pressure against the inner wall (col. 4, lines 14-17), with blood being able to flow unobstructed through the interior of the helix (column 4, lines 46-48). See also U.S. Pat. No. 5,423,865 to Bowald.
U.S. Pat. No. 5,690,686 to Min et al. describes an atrial defibrillation method in which a coronary sinus/great vein electrode is coupled to a right atrial/superior vena cava electrode and a subcutaneous electrode in the form of the housing of an implantable defibrillator. The device is stated to be preferably practiced as a combined atrial/ventricular defibrillator (col. 2, lines 26-35).
When ICDs are implanted in patients, physicians perform arrhythmia conversion testing to assure that the clinical arrhythmias can be successfully aborted by the device. In a typical clinical implant, ventricular fibrillation is electrically induced by the physician, and the ICD system is commanded to deliver a test shock of known strength after waiting about 10 seconds. If the test shock is successful, the strength is systematically reduced during subsequent trials until a strength that fails to convert the tachyarrhythmia is identified. The immediately prior shock strength that successfully converted the tachyarrhythmia is commonly referred to as the defibrillation threshold (DFT). DFTs vary among patients. The object of DFT testing at ICD implant is to identify patients that are likely not to benefit from the ICD (DFT is too high relative to maximum device output to confer a safe margin).
Technological advances have resulted in rapid evolution of ICD systems. One of the most pronounced differences in contemporary ICD systems and previous ICD systems is the reduced size of the pulse generator. Further reductions in device size may require reducing the peak voltage delivered by the device. However, to assure that nearly all patients in the ICD patient population will have DFTs lower than the maximum output, future devices will need to provide shock delivery means that substantially reduce the DFT.